a) Field of the Invention
The present invention is directed to a method and apparatus for the digitization of analog signals and a method and apparatus for data compression of analog signals.
b) Description of the Related Art
In many fields of digital information transmission there exists the common goal of transporting an initially analog source signal (measuring, studio-quality audio signal processing, and so on) to a receiver while making use of the advantages of digital transmission in order to reproduce this analog source signal in the receiver in the form of an analog output signal. For this purpose, the analog signal must be digitized and quantized on the one hand and a corresponding data compression must be carried out on the other hand in order to transmit the signal with the highest possible quality via a digital transmission system with a limited rate. After a PCM encoding, methods for data compression are usually applied which make use of the redundancy within the source signal and irrelevance with respect to specific characteristics of the consumer of the signal. The numerous existing methods for digitizing analog source signals use approaches that sometimes differ fundamentally, but all of them can be divided into two main categories:
i) Methods in which the reconstructed waveform very closely approximates the original waveform, i.e., there is no use of irrelevance. The term “waveform-preserving” is used here in place of the often applied term “lossless” with respect to the waveform coding because the digitization of an analog signal at a limited data rate is, on principle, not “lossless” (i.e., infinite entropy of a continuous random variable).
ii) Non-waveform-preserving methods. These methods are of crucial importance, for example, in the audio field, where often only the subjective aural impression at the receiver output is decisive (e.g., use of psychoacoustic masking effects). In this case, waveform changes (amplitude distortion and phase distortion) undergone by the signal through quantization and compression generally only play a subordinate role. Normally, a signal processing using irrelevance leads to a reconstructed waveform that differs greatly from the original waveform and, moreover, is often afflicted by extensive signal delay (e.g., because of spectral transformations or equivalent block-based methods). The transmission quality in a method of the type mentioned above cannot be measured by a 10 log10 signal/noise ratio (SNR) in the conventional way; rather, it must be determined by time-consuming listening tests by trained personnel. However, signal coding methods of this type are totally unusable for many areas of application (e.g., metrology, recording of waveforms for further signal processing at a later time, real-time signal transmission using digital modulation methods that do not permit significant signal delay, e.g., for wireless digital stage microphones).